Typically the substrate layers in many conventional photonic devices are uniformly and heavily doped in order to reduce series resistance and/or to improve electrical connection, especially for high-speed optical communication applications. As a result, conductive currents can cross the entire interface between the photonic device and substrate, as illustrated in FIG. 7.
However, not all the conductive currents are useful for device operation, and current from certain region even causes negative impact on device performance of the photonic device. For example, for a photonic device the useful region is the central region under coupling aperture for normal incident light. Accordingly, those conductive currents crossing other regions are regarded as noise that hampers device performance. This condition tends to worsen especially when the photonic device has a large sidewall leakage current. Thus, there is a need to solve the aforementioned problems.